I was considering how a person might get a heads up on the magnetic fields that exist in space around the galaxy. It seems to me that the best way is to measure the magnetic position and switches in stars that are near us. It is a fairly simple algorithm to look at one aspect of the data from stars and determine if there is a pattern of magnetic shifts in the available information. Clearly there are magnetic reversals and the effect would be devastating to a planetary culture that assumes a constant limited momentum change on it's surface.

The galaxy would likely have lines of force like any magnetic material due to the very way in which it operates. The end to end forces and repulsion of parallel elements leads to a thread of force which can be confined by chemical forces but collapses to lines.

The differential in local magnetic fields can be predicted by looking at the 'differential' effect and not an absolute magnetic value. Given the position of the sun with respect to the core of the galaxy, I would expect that magnetic reversals would be more common because the lines of force would become more tangled, the further we are from the center of the effect. A rotating disk of differentially charged elements would create a very complex electrical field and the strength of that field would be responsible for material becoming isolated into spiral arms.

Differential velocities in the disk would naturally produce changes in the alignment of fields and they would be stronger where matter is more dense at the center. It would seem that at least an approximation could be made by determining the confinement characteristics of the spiral arms.